1. Field of the Invention
The present invention relates to methods for determining the total metallic salt content in organic solvents when the salt is present in both organic and inorganic forms and, more particularly, is directed towards a method for determining the sodium content in fuel oils.
2. Description of the Prior Art
Generally, sodium is present as a contaminant in fuel oils. Moreover, the sodium is usually present in diverse forms simultaneously: (1) as organo-sodium salts, for example, sodium napthanates, (2) as sodium dissolved in water that is suspended in the fuel oil, (3) as solid sodium crystals dispersed within the fuel oil, and (4) as sodium absorbed onto the surface of dirt particles or present within dirt particles in the fuel oil. The quantity of the sodium present and the relative contributions of the above forms to the total quantity present depends on the geographic source of the oil, refining process, in-transient storage and handling procedures.
The total quantity of sodium in fuel oils, in whatever forms, is of considerable concern to the users of such fuel oils, particularly when they are used to power gas turbine generators. In this case, elevated sodium levels (greater than about 1 part per million (10.sup.-4 %)) cause "sulfidation", a phenomenon in which sodium and another ever-present fuel oil contaminant, sulfur, combine in a high temperature environment of the turbine engine to form highly corrosive compounds that attack the super-alloy components of the turbine engine. Sulfidation results in a loss of turbine efficiency, less operating time between costly overhauls, and, in severe cases, engine failure. Users of such equipment can take a number of well-known corrective actions to prevent or minimize sulfidation, if they can be forewarned by a reliable, on-site method for measuring the total sodium content in their fuel oil.
The presently employed methods for determining the total sodium content in fuel oils have had varying degrees of success, principally because no single method provides a reliable measurement for all of the possible forms in which the sodium may be present in the fuel oil.
Direct aspiration of a fuel oil sample into a flame photometer is unsatisfactory in most cases due to the fact that it measures only the sodium content of the fuel oil which is present as organo-sodium salts. In many fuels, organo-sodium salts represents only a few percent of the total sodium content.
Aspiration of an ultrasonically agitated fuel oil sample into a flame photometer measures the sodium dissolved in water that is suspended in the fuel oil, the organo-sodium and the crystalline sodium. This method, however, does not measure the sodium contained in solid matter such as dirt particles. Useful application of such a method is limited to filtered or centrifuged oils.
In another method, the water extraction method, the fuel oil sample is vigorously mixed for a minute or two with approximately an equal volume of sodium-free water and the layers are allowed to separate. The aqueous layer is then aspirated into a flame photometer. This method measures crystalline sodium, sodium dissolved in water in the fuel oil, and the sodium absorbed onto but not within dirt particles in the fuel oil. It also does not measure the organo-sodium content which remains behind in the fuel oil layer.